Geology of California's Imperial
Valley

A Monograph by Eugene Singer

CHAPTER
18THE
SAN
ANDREAS
FAULT
SYSTEM

The San Andreas
fault is a major geologic feature of North America.
Actually, the term San Andreas fault is a misnomer,
suggesting that the fault is a single break. It is more
accurately called the San Andreas fault system, or
zone, with several major and numerous minor branches,
particularly in southern California.

The San
Andreas Fault

Despite its
benign name (Saint Andrew), the San Andreas fault is a
violent and destructive killer. Over its long history,
spasms of its enormous energy have been released in
countless earthquakes along its great length. These have
ranged in magnitude from slight tremors to cataclysmic
upheavals and rupturing of Earth's surface.

It is a well
known and familiar geologic feature in western America. The
remarkably straight fault trace is prominent on any
California map. It looks like a knife slash from hell in a
northwest-southeast direction that attempts to separate the
Coast Ranges from central and southern
California.

The fault ranks
among the longest such structures in the world. Overland, it
is a visible break that can be traced for 625 miles from
Point Arena, north of San Francisco, to the Salton Sea, near
the border with Mexico.

The inferred
extensions of the fault are even more dramatic. Northward,
the break is thought to extend under the sea another 400
miles, possibly merging into the Murray Fracture Zone on the
deep sea floor. At the southern end, the fault is believed
to follow the axis of the Gulf of California for almost
1,000 miles.

Because of the
fault's great length, as much as 2,000 miles, movement is
intermittent as to time, and random as to place. At any
place along the fault, movement may be a one-time event, or
movement may be recurrent over thousands of years. In any
one episode, displacement may be a fraction of an inch or it
may be many feet.

The surface
trace of the fault first appears at Point Arena, a peninsula
north of San Francisco. It takes a straight path
southeastward through a series of elongated valleys to San
Francisco Bay, it crosses the bay with San Francisco on the
western block and the bay cities on the eastern or
continental block.

In the San
Francisco Bay area, the San Andreas has two major companion
branches. Generally defining the east side of the Bay is the
Hayward fault, and a few miles inland is the Calaveras
fault. The Hayward fault presently carries a high
probability forecast for an earthquake.

In an
undeviating course, the trace passes San Jose and enters the
Coast Ranges. It runs parallel to the Gabilan Range, passes
the towns of Coalinga and Parkfield (both with vivid
earthquake histories), thence it defines the elongated
Temblor Range to Tejon Pass.

This segment of
the fault in central California is very old. Between the Bay
area and the Tehachapi Mountains, rock formations appear to
have been displaced as much as 160 miles. This suggests that
the northern section of the fault may have originated about
30 to 40 million years ago, during Oligocene
time.

The fault planes
of the faults in the system are usually near vertical. In
some sections, however, the fault plane is locally tilted as
much as 60 degrees to the northeast. A local example of this
effect is the Banning fault near Cabazon in San Gorgonio
Pass. Here, the fault tilted to the point where the moving
western block slid under the eastern block, a thrust
fault.

The San Andreas
fault is a shallow fault, generally extending into the
earth's crust four to ten miles. Earthquakes associated with
shallow faults are more destructive at the surface than
those originating from deep-seated faults.

The San Andreas
is an uninterrupted zone of many braided fault traces. These
fractures branch and interlace in a swath two to eight miles
wide along the fault's great length. With these multiple
fractures creating great slices or slabs, and over countless
episodes of earth movement, the rocks within the fault zone
have been severely deformed, smashed and ground up. Thus
weakened and broken, they become subject to rapid weathering
and erosion.

So, the fault
course in central California has been eroded into a series
of elongated valleys for more than one hundred miles through
the Coast Ranges as a shallow trench only a few miles wide.
This distinctive landform was made both from erosion of the
shattered and weathered rocks and from the effect of recent
faulting. The valleys contain many typical fault-related
features such as offset streams, ponds, and scarps. This
string of continuous valleys is recognizable on any large
scale road map.

In southern
California, the fault is much younger. Major right-lateral
movement began about 12 million years ago, or during late
Miocene time.

This
considerably predates the opening of the present Gulf of
California which began about 5.5 million years ago. The
progressively younger age of segments of the fault from
north to south is consistent with ideas concerning plate
movement along the west coast of North
America.

The southern
California segment of the San Andreas fault is described
from the vicinity of Gorman, about 60 miles northwest of Los
Angeles. There, it bends abruptly to the east for six miles,
then resumes its original southeast heading.

This "Big Bend"
area is possibly the most significant tectonic area in
California today. Here, the San Andreas intersects the
left-lateral Garlock fault, the only major east-west
trending fault in southern California. Earthquakes are
common in this area. The Fort Tejon earthquake in 1857
(magnitude 8-plus) is thought to have been at least as
violent as the San Francisco earthquake 49 years later. The
San Fernando earthquake of 1971 (magnitude 6.6) was also
associated with this zone of intersection.

From Tejon Pass,
the main trace of the San Andreas fault passes through the
high desert north of Los Angeles, defining the north face of
the San Gabriel Mountains. It then separates the San Gabriel
Mountains from the San Bernardino Mountains, creating Cajon
Pass that takes Interstate 15 out of southern
California.

From the
vicinity of Cajon Pass, the southern California segment of
the San Andreas fault becomes very complex with no
distinctive single trace as in the north. Instead, it is
divided into several right-lateral elements, all somewhat
parallel to each other. Principal among these are the San
Jacinto and Elsinore faults. Several branches go through the
San Bernardino Mountains and along the north margin of San
Gorgonio Pass into the Coachella Valley.

One important
associated fault is the San Gabriel fault, defining the
southern edge of the San Gabriel Mountains in the Los
Angeles basin. This fault was probably the most active
strand for thousands of years, but there is no evidence of
recent activity. Another important branch in the Los Angeles
area is the Newport-Inglewood fault, the source of the
destructive Long Beach earthquake of 1923.

The Banning
Fault

The Banning
fault, a subordinate branch of the San Andreas, first
appears east of Riverside, trending almost due east. It
defines the north side of San Gorgonio Pass where a section
is unique in being a thrust fault. This thrust is
responsible for the low, brown-colored hills along the north
side of Interstate 10 between Cabazon and Whitewater
Canyon.

The fault has
created a distinctive gash behind these hills. In crossing
Whitewater Canyon, it forms a barrier to the passage of
groundwater down-canyon, supporting lush riparian growth in
the floor of the canyon.

The Banning
fault enters the Coachella Valley near Whitewater, crossing
Route 62 about 1.5 miles north of the interchange with
Interstate 10. It intersects Indian Avenue one mile north of
the freeway, and crosses Palm Drive at 20th Avenue, then
continues to the southwest past Palm Springs.

The surface
trace of the San Andreas fault is conspicuous on the valley
floor by the lineup of vegetation along its north side, a
result of fault-dammed groundwater flow. This prominent
feature is conpicuous as viewed from the observation post at
the top of the aerial tramway.

The Banning
fault trace defines the southern margin of the Indio Hills,
and supports several beautiful palm oases in the canyons
where ground water reaches the surface as seepage along the
fault trace. Similarly, the northern margin of the Indio
Hills is defined by the Mission Creek fault, and here, too,
may be found palm oases, principally Thousand Palms
Oasis.

While the
Banning fault is very well defined by surface features, its
relationship to the main San Andreas fault has not been
firmly established. The area in question is the complex
fault geology of San Gorgonio Pass.

The Mission
Creek Fault

About six miles
north of the Banning fault, another branch, the Mission
Creek fault, enters the Coachella Valley near the mouth of
Morongo Valley. The Mission Creek fault passes directly
through the town of Desert Hot Springs, and is responsible
for the many warm springs in that area. It intersects Dillon
Road near Wide Canyon Road, and marks the northern margin of
the Indio Hills. Thousand Palms Oasis is on this
fault.

Locally, the
Mission Creek fault is the most active branch in the system.
Seismological data relating to recent earthquakes and
measures from precise triangulation surveys show current
persistent movement. Disrupted alluvial deposits, truncated
older alluvial fans, and vegetation lines are evidence of
its trace. In particular, low scarps may be seen where
Indian Avenue meets Route 62, and again at Miracle Hill,
east of Desert Hot Springs.

The Banning
fault and the Mission Creek fault join at Biskra Palms in
the Indio Hills near the north end of Madison
Street).

The San Andreas
fault continues along the northwest shore of Salton Sea. At
Salt Creek Wash, the surface trace of the great fault ends,
more than 600 miles from its northern end.

Current thinking
is that the San Andreas fault, buried under alluvium, merges
with the Imperial fault. It then continues southeastward to
the Gulf of California, defining the western margin of the
Gulf, finally merging into the East Pacific Rise.

The South
Pass Fault

The inferred
South Pass fault defines the southern margin of San Gorgonio
Pass along the edge of the San Jacinto mountains. The fault
has no surface trace, being deeply buried in alluvium. The
principal rationale for placing a fault here is (1) the
uniform straight mountain scarp, and (2) without a fault, it
would be difficult to account for the pass structure. Where
the fault enters the Coachella Valley, it curves to the
southwest, past Windy Point to about Chino Canyon.

The San
Jacinto Fault

The most
seismically active fault in southern California today is the
San Jacinto fault and its branches. It is also the fault
that is potentially troublesome to the Salton Trough. Many
earthquakes within historic time have been associated with
it.

The San Jacinto
fault is a major element of the San Andreas system.
Separating from the San Andreas west of Cajon Pass, it takes
a more southerly but generally parallel track. The trace
bisects the city of San Bernardino, and passes directly
under the four-level freeway interchange at Colton. Its path
takes it behind the San Jacinto Mountains, passing the
communities of San Jacinto and Hemet. Along the mountain
front in this area, the fault has dammed groundwater
channels, forcing water to the surface as hot
springs.

It lies west and
south of the San Jacinto Mountains, and for several miles
forms the northeast edge of Borrego Valley. The fault cuts
the Ocotillo Badlands near Ocotillo Wells, then enters the
Imperial Valley where its trace is buried under recent lake
bed sediments. The known length of the fault is
approximately 180 miles.

Like the San
Andreas, it is a complex system of many faults with local
names. At its northwest end, following its separation from
the San Andreas fault near San Bernardino, it generally has
one or more well-defined breaks, especially where its trace
cuts across rocks of somewhat recent age. Southeastward, it
includes several zones of subparallel breaks, separating
masses of bed rock into slabs ranging in width from a few
feet to thousands of feet. Although they are parts of the
San Jacinto fault zone, many of these subsidiary faults are
themselves major features and commonly are given individual
names. One important subordinate fault is the Thomas
Mountain fault that forms the southwest margin of Garner
Valley.

This pattern of
multiple faults continues to broaden to the southeast where
the San Jacinto zone enters the Salton Trough. There, at
least six subordinate faults are spaced half a mile to three
miles apart. This pattern continues through the Imperial
Valley and into Baja California.

The major breaks
in the San Jacinto fault zone dip very steeply to nearly
vertical, and the entire zone appears to be a very
deep-rooted feature.

There is
abundant evidence of recency of movement along most of the
length of the fault. In its central section, along the San
Jacinto Mountains and the Santa Rosa Mountains, there are
many fault scarps, elongate trenches, sag ponds, aligned
canyons, and offset drainage. Many earthquakes, moderate to
severe in scale, have been recorded from the City of San
Jacinto south into Mexico.

The most active
branch at this time is the Imperial fault, the source of
many tremors and quakes in recent years, some of them very
destructive. Another active branch is the Superstition Hills
fault along the west side of the Imperial Valley. The very
active Cerro Prieto fault on the Mexican side of the border
is a member of the San Jacinto system, and is probably an
extension of the Imperial fault.

The San Jacinto
fault is a young, right lateral zone of seismic strain that
has dominated fault movement in southern California for a
least a century. Notwithstanding the notoriety of the San
Andreas fault, since 1857 there have been 36 major
earthquakes identified to faults in the San Jacinto system.
Of these, 15 have originated along the Imperial fault in the
Salton Trough.

Between 1915 and
1954, five historic large quakes, all with magnitudes
between 6.0 to 6.8 on the Richter scale, occurred along this
fault between the City of San Jacinto and the Salton
Sea.

While presently
more active than the San Andreas fault, the San Jacinto
fault is much younger in age, having slipped laterally only
about 15 miles, compared to nearly 200 miles of displacement
along the San Andreas fault. The age of the San Jacinto
fault is uncertain, but it seems probable that the fault has
been active since at least early Tertiary time.

The
Elsinore Fault

A brief mention
of the Elsinore Fault is made here only because it is one
arm of a trilateral split of the San Andreas Fault. As a
geographic feature it is not associated with the Salton
Valley.

The third major
fault in the southland is the Elsinore fault, also a member
of the San Andreas system. Its trace is parallel to the San
Jacinto, and about 20 miles west. The fault originates
beneath the alluvium of the Los Angeles Basin. Running
southeast, it sharply defines Elsinore Valley and Lake
Elsinore). Passing south of the Vallecito Mountains
bordering Borrego Valley, the fault enters the Salton Trough
near the Coyote Mountains in the extreme southwest corner of
the Imperial Valley.

Of the three
principal branches including the San Andreas and the San
Jacinto faults the Elsinore fault has been considerably less
active in historic time.

Movement
along the San Andreas fault

The San Andreas
is a right lateral fault, with the relative movement of
adjacent land masses being sideways. Along the San Andreas
system, movement of the western block has been consistently
to the northwest. Over millions of years, the cumulative
displacement amounts to several hundred miles. Or, as Armand
Eardley put it, "The San Andreas marks such an important
contact that rarely can it be crossed, except in recent
alluvium, without passing into significantly different
rocks.

Displacement, or
the amount of movement along lateral faults is determined by
identifying distinctive rock units found on opposite sides
of a fault, but are some distance apart. One such technique
is identification of rock debris in a sedimentary basin that
could not possibly have come from the local mountains. The
geologist then attempts to match up the sedimentary material
with the appropriate source on the other side of the
fault.

A local example
of this occurs in Whitewater Canyon. A rock formation known
as the Coachella Fanglomerate a coarse, bouldery ancient
alluvial fan deposit is exposed where the Mission Creek
fault crosses the canyon. Geologists have studied the
boulder fragments in the Coachella Fanglomerate in detail,
finding several distinctive types south of the Mission Creek
fault that do not appear north of the fault. These rock
fragments appear to have been derived from a source area
close to the Cargo Muchacho Mountains, near the Mexican
border. Such a correlation requires 130 miles of
right-lateral separation within the San Andreas fault
system.

Studies at many
locations between Soledad Pass and the Salton Sea have
established that the San Andreas fault has a total offset of
at least 140 miles. As the southern section of the San
Andreas fault can be dated back to the Miocene Epoch, about
12 million years ago, this suggests an average displacement
of more than one inch per year.

Offset of the
rocks has also been observed at many points along the length
of the San Andreas fault. Older rocks appear to have been
displaced more than 350 miles, while progressively younger
rocks are displaced progressively fewer
miles.

It is difficult
to absorb the scale on which nature works! A displacement of
hundreds of miles along the San Andreas fault can only be
accepted when we understand that it is the result of an
immense number of small steps occurring over a vast span of
time.